Why was Herbert Hoover not reelected?

The timing of Hoover's presidency was unfortunate. Elected in 1928, he took office in March of 1929, shortly before the stock market crash of October of 1929 and the Great Depression that ensued.
Hoover lacked the governmental machinery to respond to the Great Depression in an adequate way. Before Franklin D. Roosevelt took office, the government lacked the ability to respond in a comprehensive way to the far-reaching financial effects of the Great Depression. Instead, Hoover responded by setting up the Reconstruction Finance Corporation (RFC), which made loans to businesses—rather than directly helping people.
However, the Great Depression inflicted a lot of suffering on Americans, and they were eager for more help. In 1932, members of the so-called "Bonus Army"—composed of World War I veterans calling for their bonuses (promised in 1945) to be paid early—were forcibly removed from their encampments in Washington, DC, by the military. This event made people believe that Hoover was not sensitive to their suffering during the Great Depression, paving the way for Roosevelt's victory in 1932.